1. Field of the Invention
The present invention relates to a steering control method for vehicles—especially electric vehicles—having four-wheel-independent steering. In particular, it relates to a vehicle steering control method for safely and smoothly operating a vehicle in a steering mode adapted to the constraints of passageways in facilities in which the vehicle is being driven, and to the placement of objects adjacent to those passageways. In the present application, the term ‘steering mode’ denotes a basic pattern of travel paths described by the wheels of a vehicle being steered.
2. Background Art
Previously, in JP Patent Application No. 2001-351127, the inventors proposed a method for controlling operation of an electric vehicle with four-wheel-independent steering for steering in a wide variety of indoor/outdoor facilities such as hospitals, nursing homes, supply distribution bases, computer warehouses, large commercial facilities, libraries, sports/entertainment complexes, amusement parks, etc., by using steering modes adapted to conform to the constraints associated with the passageways in such indoor/outdoor facilities, and to the placement of objects in close proximity to the passageways.
The specification of that prior JP Patent Application No. 2001-351127 describes how the steering of an electric vehicle in which steering/drive control is performed through separate drive motors and steering motors for each of its four wheels (left front, right front, left rear and right rear) in a number of different steering modes M1, M2, M3, M4, and M5. The specification describes, as examples of steering modes, an M1 steering mode wherein the travel paths of the right and left rear wheels follow in the paths of the right and left front wheels; an M2 steering mode wherein the travel paths of the front and rear wheels run parallel to each other; an M3 steering mode wherein the rear wheel turning path relative to the front wheel turning path is an ‘inside wheel difference path;’ an M4 steering mode wherein the vehicle turns to the right around the right rear wheel as the center of rotation of the turn, and turns to the left around the left rear wheel as the center of rotation of the turn; and an M5 steering mode wherein the vehicle turns to the right with the right front wheel as the center of rotation of the turn, and turns to the left with the left front wheel as the center of rotation of the turn. Also proposed in this application is a steering control method wherein a mode is selected from the M1, M2, M3, M4 and M5 steering modes as the mode best adapted to the conditions of the passageway to be driven; and the rotation of each steering motor and each drive motor then is controlled such that the steering angles α1, α2, α3, α4, and rotation speeds n1, n2, n3, n4, of the respective wheels conform to ‘steering constraint condition equations’ (hereinafter shortened to ‘condition equation’) that define the conditions that must exist in order to execute the selected prescribed steering mode.
The following equations were proposed as steering constraint condition equations (condition equations) for the above steering modes.
For the M1 steering mode:
                              α          1                =                              -                          α              3                                =                                    tan                              -                1                                      ⁡                          (                              L                                  R                  -                  W                                            )                                                          (        E11        )                                          α          2                =                              -                          α              4                                =                                    tan                              -                1                                      ⁡                          (                              L                                  R                  +                  W                                            )                                                          (        E12        )            n1:n2:n3:n4=√{square root over ((R−W)2+L2)}:√{square root over ((R+W)2+L2)}:√{square root over ((R−W)2+L2)}:√{square root over ((R+W)2L2)}  (E13)
For the M2 steering mode:α1=α2=α3α4  (E21)n1=n2=n3=n4  (E22)
For the M3 steering mode:
                              α          1                =                              tan                          -              1                                ⁡                      (                                          2                ⁢                L                                            R                -                W                                      )                                              (        E31        )                                          α          2                =                              tan                          -              1                                ⁡                      (                                          2                ⁢                L                                            R                +                W                                      )                                              (        E32        )            α3=α4=0  (E33)n1:n2:n4=√{square root over ((R−W)2+(2L)2)}{square root over ((R−W)2+(2L)2)}:√{square root over ((R+W)2+(2L)2)}{square root over ((R+W)2+(2L)2)}:|R−W|:|R+W|  (E34)
In the M4 steering mode, for a right turn:
                              α          1                =                  π          2                                    (        E41        )                                          α          2                =                              tan                          -              1                                ⁡                      (                          L              W                        )                                              (        E42        )            α3=α4=0  (E43)n1:n2:n3:n4=L:√{square root over (W2+L2)}0:W  (E44)
and for a left turn:
                              α          1                =                              tan                          -              1                                ⁡                      (                          -                              L                W                                      )                                              (        E45        )                                          α          2                =                  -                      π            2                                              (        E46        )            α3=α4=0  (E47)n1:n2:n3:n4=√{square root over (W2+L2)}L:W:0  (E48)
In the M5 steering mode, for a right turn:α1=α2=0  (E51)
                              α          3                =                  -                      π            2                                              (        E52        )            
                              α          4                =                              tan                          -              1                                ⁡                      (                          -                              L                W                                      )                                              (        E53        )            n1:n2:n3:n4=0:W:L√{square root over (W2+L2)}  (E54)
and for a left turn:α1=α2=0  (E55)
                              α          3                =                              tan                          -              1                                ⁡                      (                          L              W                        )                                              (        E56        )                                          α          4                =                  π          2                                    (        E57        )            n1:n2:n3:n4=W:0:√{square root over (W2+L2)}:L  (E58)
where, in the above equations,    α1 is the steering angle for the right front wheel,    α2 is the steering angle for the left front wheel,    α3 the steering angle for the right rear wheel,    α4 is the steering angle for the left rear wheel,    n1 is the rotation speed for the right front wheel,    n2 is the rotation speed for the left front wheel,    n3 is the rotation speed for the right rear wheel,    n3 is the rotation speed for the left rear wheel,    L is the distance between each wheel and a center-line X between the front and rear wheels,    W is the distance between each wheel and a center-line Y between the left and right wheels, and    R, for the case in which the turning paths of the wheels inscribe concentric arcs, is the distance between the center of the concentric arcs and a point central to the locations of the four wheels (the distance from the center of the vehicle to the vehicle's center of rotation; i.e., the radius of vehicle rotation).
The problem with this, however, is that to change the vehicle's direction of travel, a steering command is issued with the distance R (vehicle turning radius) as a steering command value for increasing or decreasing the R setting of the existing steering command value, thus to cause the steering angles of the wheels (α1, α2, α3, α4) to increase or decrease as required to conform to the above equations. When the steering command value R is changed from an existing value R1 to a desired value R2, however, it takes a small amount of time (‘steering command lag time’) before the wheel steering angles (α1, α2, α3, α4) reach the new values required to satisfy the above equations. In the steering process, during this steering command lag time, the left and right wheels may point [severely] outward (toe-out) or [severely] inward (toe-in) with respect to the vehicle's direction of travel. (These toe-out and toe-in phenomena are referred to in general as the ‘toe-in/out phenomenon’) If [severe] toe-in/out occurs during the steering process, it can apply enough force to the steering mechanism to damage it. It can also destabilize objects and people riding in or on the vehicle, creating a dangerous situation.
For a driver actually operating this vehicle, it would be difficult to grasp, in an intuitive or physical sense, how the distance R is related to the steering angles of the wheels (direction of travel of the vehicle). Moreover, the value of the distance R can invert, going abruptly from −∞ to +∞ (full left to full right), or from +∞ to −∞ (full right to full left) with the direction of travel of the vehicle as the left/right dividing line. Therefore, from the standpoint of practical driving considerations, it would not be desirable to use such discontinuous changes in the distance R as a steering angle setting parameter for setting steering command values for the steering angles α1, α2, α3, α4.